User equipment

ABSTRACT

Techniques are disclosed for avoiding reduction in throughput due to redundant transmission of downlink packets when a split bearer is deleted. A user equipment includes a transmission and reception unit to communicate with a master base station and a secondary base station simultaneously in dual connectivity; an RLC (Radio Link Control) layer processing unit configured to have an RLC layer for the master base station and an RLC layer for the secondary base station; and a PDCP (Packet Data Convergence Protocol) layer processing unit configured to have a PDCP layer transmitting and receiving data to/from the RLC layer for the master base station and the RLC layer for the secondary base station. When the split bearer configured for the secondary base station is deleted, the RLC layer processing unit performs re-establishment on the RLC layer for the secondary base station and releases the split bearer after execution of the re-establishment.

TECHNICAL FIELD

The present invention relates to a radio communication system.

BACKGROUND ART

Presently, as a next-generation communication standard of LTE (Long TermEvolution) systems, enhancement of LTE-Advanced is promoted. InLTE-Advanced systems, a carrier aggregation (CA) technique is introducedto achieve a higher throughput than that of LTE systems while ensuringbackward compatibility with the LTE systems. In the carrier aggregation,a component carrier (CC) having the maximum bandwidth of 20 MHzsupported by the LTE systems is used as a basic component, and it isdesigned to achieve communication in a broader band by using thesemultiple component carriers simultaneously.

In the carrier aggregation, user equipment (UE) can use multiplecomponent carriers simultaneously to communicate with a base station(evolved NodeB: eNB). In the carrier aggregation, a highly reliableprimary cell (PCell) to ensure connectivity to the user equipment and asecondary cell (SCell) or a secondary cell group (SCG) additionallyconfigured for the user equipment in connection with the primary cellare configured.

The primary cell is a cell similar to a serving cell in the LTE systemsand serves as a cell to ensure connectivity between the user equipmentand a network. On the other hand, the secondary cell or the secondarycell group is a cell or a cell group configured for the user equipmentin addition to the primary cell.

In the carrier aggregation up to LTE Release 10 (Rel-10), as illustratedin the left side in FIG. 1, it is defined that user equipment usesmultiple component carriers served from a single base station to conductsimultaneous communication. Meanwhile, in Rel-12, the carrieraggregation in Rel-10 is further enhanced, and as illustrated in theright side in FIG. 1, dual connectivity (DC) where the user equipmentuses multiple component carriers served from multiple base stations toconduct the simultaneous communication is discussed. For example, if allcomponent carriers cannot be accommodated in a single base station, itis considered that the dual connectivity can be effectively utilized toachieve a throughput nearly equal to that in Rel-10.

In the dual connectivity, a split bearer is configured. In the casewhere a master base station or a macro base station (MeNB) is used as ananchor node for distributing the bearer, as illustrated in FIG. 2, themaster base station distributes downlink packets received from a S-GW(Serving Gateway) into packets for transmission to the user equipmentvia its own cell and packets for transmission to the user equipment viaa secondary base station (SeNB). If the split bearer using the masterbase station as the anchor node is configured, as illustrated in FIG. 3,the user equipment has a physical layer (PHY), a MAC (Medium AccessControl) layer (m-MAC) and an RLC (Radio Link Control) layer (m-RLC) forthe master base station; a PHY layer, a s-MAC layer and a s-RLC layerfor the secondary base station; and a PDCP layer coupled to the m-RLClayer and the s-RLC layer. Packets received from the master base stationand packets received from the secondary base station are reordered atthe PDCP layer, which are then delivered to an upper layer.

Also, according to LTE standard, re-establishment is performed on theRLC layer and the PDCP layer in handover and reconnection. In there-establishment of the RLC layer, the transmitting side in the RLClayer discards all of to-be-transmitted RLC PDUs (Packet Data Units)while the receiving side in the RLC layer re-assembles as many RLC SDUs(Service Data Units) as possible from the received RLC PDUs and deliversthe re-assembled RLC SDUs to the PDCP layer. Also, various timers foruse in the RLC layer are stopped and reset, and all various variablesare initialized.

On the other hand, in the re-establishment of the PDCP layer, thetransmitting side in the PDCP layer retransmits PDCP PDUs, whoseacknowledgement (ACK) has not been received at the RLC layer, while thereceiving side in the PDCP layer reorders RLC SDUs received with there-establishment of the RLC layer and RLC SDUs newly transmitted afterexecution of the re-establishment. Here, the PDCP layer performsdifferent control operations with a reception window for reordering onthe RLC SDUs received with the re-establishment of the RLC layer and thenewly transmitted RLC SDUs. Specifically, the PDCP layer does not updatethe reception window for the RLC SDUs received with the re-establishmentin the RLC layer if PDCP sequence numbers of the received packets areout of sequence, but the PDCP layer updates the reception window for thenewly transmitted RLC SDUs even if the PDCP sequence numbers of thereceived packets are out of sequence.

See 3GPP R2-131782 in details, for example.

SUMMARY OF INVENTION Problem to be Solved by the Invention

In cases where the split bearer is deleted in dual connectivity, theabove-stated operations for handover and reconnection are not performedpresently. For example, as illustrated in FIG. 4, when the split beareris deleted in downlink communication, all RLC PDUs #1 to #4, which arewaiting for reordering due to no reception of RLC PDU #0 at the s-RLClayer, are discarded. As a result, after the split bearer is released,the master base station needs to retransmit not only thereception-waiting RLC PDU #0 but also the RLC PDUs #1 to #4 received atthe s-RLC layer in the user equipment, which reduces throughput.Although downlink packets can be transmitted in the split bearer fromboth the master base station and the secondary base station, the casewhere packets are transmitted from only the secondary base station isillustrated in FIG. 4 for ease in description.

Also, in the split bearer, uplink communication from the user equipmentis conducted by switching between two transmission directions,transmission to the master base station and transmission to thesecondary base station, by RRC signaling. At switch timings of theuplink transmission direction, the re-establishment is not presentlyperformed on the PDCP layer and the RLC layer. As a result, asillustrated in FIG. 5, for example, PDCP PDUs #0 to #3 distributed fortransmission to the secondary base station in the uplink communicationare still transmitted to the secondary base station even after changingthe uplink transmission direction from the secondary base station to themaster base station. On the other hand, after the uplink transmissiondirection change command, the secondary base station may stop receptionfrom the user equipment, and RLC PDUs #0 to #3 generated from the PDCPPDUs #0 to #3 would be unnecessarily transmitted. For ease ofdescription, PDCP PDU:RLC PDU=1:1 is set in FIG. 5. If the userequipment transmits packets subsequent to the RLC PDU #4 to thepost-changed master base station but the secondary base station has notreceived the RLC PDUs #0 to #3, the master base station would receivethe RLC PDU #4 before the RLC PDUs #0 to #3. Since the re-establishmentis not conducted, the PDCP layer in the master base station woulddiscard the subsequently arriving RLC PDUs #0 to #3 as being out of thereception window.

In light of the above-stated problem, an object of the present inventionis to provide some techniques for avoiding reduction in throughput dueto redundant transmission of downlink packets in split bearer deletion.

Means for Solving the Problem

In order to achieve the above object, one aspect of the presentinvention relates to user equipment, comprising: a transmission andreception unit configured to communicate with a master base station anda secondary base station simultaneously in dual connectivity; an RLC(Radio Link Control) layer processing unit configured to have an RLClayer for the master base station and an RLC layer for the secondarybase station; and a PDCP (Packet Data Convergence Protocol) layerprocessing unit configured to have a PDCP layer transmitting andreceiving data to/from the RLC layer for the master base station and theRLC layer for the secondary base station, wherein when a split bearerconfigured for the secondary base station is deleted, the RLC layerprocessing unit performs re-establishment on the RLC layer for thesecondary base station and releases the split bearer after execution ofthe re-establishment.

Advantage of the Invention

According to the present invention, it is possible to avoid reduction inthroughput due to redundant transmission of downlink packets in splitbearer deletion.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram for illustrating carrier aggregation;

FIG. 2 is a schematic diagram for illustrating split bearer operationswhere a macro base station serves as an anchor node;

FIG. 3 is a diagram for illustrating a layer structure for downlinkcommunication in configured split bearer;

FIG. 4 is a schematic diagram for illustrating conventional split bearerdeletion operations;

FIG. 5 is a schematic diagram for illustrating conventional uplinktransmission direction change operations;

FIG. 6 is a schematic diagram for illustrating a radio communicationsystem according to one embodiment of the present invention;

FIG. 7 is a block diagram for illustrating an arrangement of userequipment according to one embodiment of the present invention;

FIG. 8 is a schematic diagram for illustrating split bearer deletionoperations according to a first embodiment of the present invention;

FIG. 9 is a schematic diagram for illustrating conventional reorderingoperations at a PDCP layer in RLC layer re-establishment;

FIG. 10 is a schematic diagram for illustrating reordering operations atthe PDCP layer in RLC layer re-establishment according to the firstembodiment of the present invention;

FIG. 11 is a flowchart for illustrating split bearer deletion in an RLClayer processing unit according to the first embodiment of the presentinvention;

FIG. 12 is a flowchart for illustrating split bearer deletion in a PDCPlayer processing unit according to the first embodiment of the presentinvention;

FIG. 13 is a flowchart for illustrating uplink transmission directionchange operations in user equipment according to a second embodiment ofthe present invention; and

FIG. 14 is a block diagram for illustrating an arrangement of a basestation according to the second embodiment of the present invention.

EMBODIMENTS OF THE INVENTION

Embodiments of the present invention are described below with referenceto the drawings.

In embodiments as stated below, user equipment and base stationssupporting dual connectivity are disclosed. Summarizing embodiments asstated below, in downlink communication in the split bearer, when asplit bearer configured for a secondary base station is deleted, theuser equipment performs re-establishment on an s-RLC layer, and thes-RLC layer assembles as many RLC SDUs as possible without discardingRLC PDUs waiting for reordering and transmits them to a PDCP layer. Inaddition, the user equipment performs re-establishment on the PDCPlayer, and even if out-of-sequence in RLC SDUs (PDCP PDUs) receivedbefore expiration of a reordering timer is not eliminated, the PDCPlayer uses the reordering timer to transmit the PDCP PDUs waiting forreordering to an upper layer. As a result, even if the missing RLC PDUsare not transmitted to the user equipment due to discarding or others,it is possible to avoid the situation where the PDCP PDUs waiting forreordering cannot be transmitted to the upper layer.

Also, in another embodiment, when the transmission direction of uplinkdata packets in uplink communication is changed in the split bearer, theuser equipment performs the re-establishment on the RLC layer and thePDCP layer. As a result, the user equipment can stop transmitting datapackets distributed to a pre-changed transmission direction and transmituntransmitted data packets to a post-changed base station in sequence.

A radio communication system according to one embodiment of the presentinvention is described with reference to FIG. 6. FIG. 6 is a schematicdiagram for illustrating a radio communication system according to oneembodiment of the present invention.

As illustrated in FIG. 6, a radio communication system 10 has userequipment 100 and base stations 200A, 200B. The radio communicationsystem 10 supports dual connectivity where the user equipment 100 usescomponent carriers CC#1, CC#2 served from the multiple base stations200A, 200B to conduct simultaneous communication, and as illustrated,the user equipment 100 uses a dual connectivity function to communicatewith the master base station (MeNB) 200A and the secondary base station(SeNB) 200B. In the illustrated embodiment, only the two base stations200A, 200B are illustrated, but a large number of base stations 200 aregenerally disposed to cover a service area of the radio communicationsystem 10.

The user equipment 100 has the dual connectivity function to communicatewith the multiple base stations 200A, 200B simultaneously. Typically, asillustrated, the user equipment 100 may be any appropriate informationprocessing device with a radio communication function such as asmartphone, a mobile phone, a tablet and a mobile router. The userequipment 100 is arranged with a CPU (Central Processing Unit) such as aprocessor, a memory device such as a RAM (Random Access Memory) and aflash memory, a radio communication device for transmitting andreceiving radio signals to/from the base stations 200A, 200B and so on.For example, functions and operations of the user equipment 100 asstated below may be implemented by the CPU processing and running dataand programs stored in the memory device. However, the user equipment100 is not limited to the above-stated hardware configuration and may bearranged with circuits for implementing one or more of operations asstated below.

The base stations 200A, 200B (which may be collectively referred to asthe base stations 200 hereinafter) establish a radio connection to theuser equipment 100 to transmit downlink (DL) packets received fromnetwork devices, such as an upper station and a server, communicativelyconnected on a core network (not shown) to the user equipment 100 aswell as transmit uplink (UL) packets received from the user equipment100 to the network devices. In the illustrated embodiment, the basestation 200A serves as a master base station (MeNB) or a primary basestation, and the base station 200B serves as a secondary base station(SeNB). In the dual connectivity, the master base station 200A controlssimultaneous communication between the user equipment 100 and the basestations 200A, 200B in accordance with the dual connectivity andcontrols communication with the upper core network (not shown). The basestation 200 is typically arranged with hardware resources such as anantenna for transmitting and receiving radio signals to/from the userequipment 100, a communication interface for communicating with anadjacent base station 200, a processor and a circuit for processingsignals transmitted and received to/from the user equipment 100 and theadjacent base station 200. Functions and operations of the base station200 as stated below may be implemented by the processor processing andrunning data and programs stored in the memory device. However, the basestation 200 is not limited to the above-stated hardware configurationand may have any other appropriate hardware configuration.

Next, an arrangement of the user equipment according to one embodimentof the present invention is described with reference to FIG. 7. FIG. 7is a block diagram for illustrating an arrangement of the user equipmentaccording to one embodiment of the present invention.

As illustrated in FIG. 7, the user equipment 100 has a transmission andreception unit 110, an RLC layer processing unit 120 and a PDCP layerprocessing unit 130.

The transmission and reception unit 110 communicates with the masterbase station 200A and the secondary base station 200B simultaneously indual connectivity. Specifically, the transmission and reception unit 110transmits and receives various radio channels, such as uplink/downlinkcontrol channels and uplink/downlink data channels, to/from the masterbase station 200A and the secondary base station 200B.

The RLC layer processing unit 120 has an RLC (m-RLC) layer 121 for themaster base station 200A and an RLC (s-RLC) layer for the secondary basestation 200B. In downlink communication in the split bearer, the m-RLClayer 121 assembles RLC SDUs from RLC PDUs received from the master basestation 200A and transmits them to the PDCP layer processing unit 130.On the other hand, the s-RLC layer 122 assembles RLC SDUs from packets(RLC PDUs) received from the secondary base station 200B and transmitsthem to the PDCP layer processing unit 130. Also, in uplinkcommunication in the split bearer, the m-RLC layer 121 assembles RLCPDUs from PDCP PDUs received from the PDCP layer processing unit 130 andtransmits them to the master base station 200A via a lower layer (notshown). On the other hand, the s-RLC layer 122 assembles RLC PDUs frompackets PDCP PDUs received from the PDCP layer processing unit 130 andtransmits them to the secondary base station 200B via the lower layer(not shown).

The PDCP layer processing unit 130 has a PDCP layer 131 transmitting andreceiving data to/from the m-RLC layer 121 for the master base station200A and the s-RLC layer 122 for the secondary base station 200B. Indownlink communication in the split bearer, the PDCP layer 131 receivesRLC SDUs from the m-RLC layer 121 and the s-RLC layer 122, reorders thereceived packets based on sequence numbers (SNs) of the respectivepackets and transmits the reordered packets to an upper layer (notshown). In the reordering, the PDCP layer processing unit 130 uses areception window. For packets received from the RLC layers withre-establishment, if the sequence numbers are out of sequence, the PDCPlayer processing unit 130 does not update the reception window. Forpackets newly transmitted rather than with the re-establishment, on theother hand, even if the sequence numbers are out of sequence, the PDCPlayer processing unit 130 determines that the packets have beendiscarded at the base station 200A or 200B serving as the transmittingside and updates the reception window. Also, in uplink communication inthe split bearer, the PDCP layer 131 distributes to-be-transmitted PDCPPDUs into packets to be transmitted to the master base station 200A andpackets to be transmitted to the secondary base station 200B andtransmits the distributed packets to the m-RLC layer 121 and the s-RLClayer 122.

Next, split bearer deletion according to the first embodiment of thepresent invention is described with reference to FIGS. 8-12. As statedabove with reference to FIG. 4, in conventional split bearer deletion,all packets waiting for reordering at the s-RLC layer 122 are discardedbefore deletion, and after the split bearer for the secondary basestation 200B is deleted, the master base station 200A must retransmitthe discarded packets. According to the first embodiment of the presentinvention, as illustrated in FIG. 8, before deletion of the splitbearer, the RLC layer processing unit 120 assembles as many RLC SDUs aspossible from the packets (RLC-PUDs) waiting for reordering at the s-RLClayer 122 and transmits the RLC SDUs to the PDCP layer processing unit130. As a result, after deletion of the split bearer for the secondarybase station 200B, the master base station 200A has to retransmit onlythe packets waiting for reception, which can avoid reduction inthroughput.

In the first embodiment, when the split bearer configured for thesecondary base station 200B is deleted, the RLC layer processing unit120 performs re-establishment on the s-RLC layer 122 for the secondarybase station 200B and releases the split bearer after execution of there-establishment. For example, in the LTE standard, upon receiving asplit bearer deletion command from the master base station 200A or thesecondary base station 200B, the RLC layer processing unit 120 performsre-establishment on the s-RLC layer 122 configured for a secondary cellgroup (SCG) and then releases the split bearer.

In one embodiment, the RLC layer processing unit 120 may transmit datapackets staying in the s-RLC layer 122 for the secondary base station200B or the SCG to the PDCP layer processing unit 130 in there-establishment. For example, the staying data packets are RLC PDUswaiting for reordering at the s-RLC layer 122 for the secondary basestation 200B or the SCG, and the RLC layer processing unit 120 mayassemble RLC SDUs from the RLC PDUs waiting for reordering and transmitthe RLC SUDs to the PDCP layer processing unit 130. When the RLC SDUsare assembled from the RLC PDUs waiting for reordering, similar tore-establishment for handover or reconnection, the RLC layer processingunit 120 re-assembles as many RLC SDUs as possible from the RLC PDUswaiting for reordering and delivers the re-assembled RLC SDUs to thePDCP layer processing unit 130.

Also, in one embodiment, if releasing the split bearer is deletion on aper EPS (Evolved Packet System) bearer basis, the RLC layer processingunit 120 may delete the indicated EPS bearer without execution ofre-establishment for the s-RLC layer 122 for the secondary base station200B or the SCG. Specifically, in the LTE standard, if releasing thesplit bearer is deletion on a per-EPS bearer basis, the RLC layerprocessing unit 120 may not perform re-establishment on the s-RLC layer122 for the secondary base station 200B or the SCG to re-assemble RLCSDUs from RLC PDUs waiting for reordering and transmit the RLC SDUs tothe PDCP layer processing unit 130.

As stated above, when the split bearer is deleted, the RLC layerprocessing unit 120 performs the re-establishment on the s-RLC layer 122and re-assembles as many RLC SDUs as possible from the RLC PDUs withoutdiscarding the RLC PDUs waiting for reordering. Then, the conventionalPDCP layer changes operations for the received PDCP PDUs depending onoperations at an lower layer and performs different operations, forexample, handling missing packets as packets waiting for reordering ordiscarding the missing packets. In the above-stated case where there-establishment is performed on the s-RLC layer 122, when thereordering is performed similar to the conventional one, unnecessarywaiting for reordering would arise. For example, as illustrated in FIG.9, if the PDCP layer processing unit 130 receives re-assembled RLC SDUs#1 to #4 from the s-RLC layer 122, the PDCP layer processing unit 130waits for reception of RLC SDU #0 retransmitted from the master basestation 200A for reordering. If the master base station 200A discardsthe RLC SDU #0, the PDCP layer processing unit 130 cannot receive theRLC SDU #0 and accordingly deliver the RLC SDUs #1 to #4 waiting forreordering to an upper layer. Accordingly, it is undesirable fromstandpoints of performance that in the case where the re-establishmentis performed on the s-RLC layer 122, the PDCP layer processing unit 130performs the reordering similar to the conventional manner.

To this end, according to the first embodiment of the present invention,the PDCP layer processing unit 130 has a reordering timer activated inresponse to detecting that data packets are out of sequence, and usesthe reordering timer to control reordering of packets received from theRLC layer processing unit 120 regardless of whether the re-establishmenthas been performed on the s-RLC layer 122. Specifically, upon expirationof the reordering timer, the PDCP layer processing unit 130 transmitsdata packets, which are waiting for reordering and are received from theRLC layer processing unit 120, to an upper layer. For example, asillustrated in FIG. 10, if the PDCP layer processing unit 130 receivesRLC SDUs #1 to #4 from the s-RLC layer 122, the PDCP layer processingunit 130 detects that RLC SDU #0 is missing and activates the reorderingtimer. If the PDCP layer processing unit 130 has failed to receive theRLC SDU #0 by expiration of the reordering timer, the PDCP layerprocessing unit 130 delivers the RLC SDUs #1 to #4 waiting forreordering to an upper layer. As a result, even in the case where themaster base station 200A has discarded the RLC SDU #0, upon expirationof the reordering timer, the PDCP layer processing unit 130 can deliverthe RLC SDUs #1 to #4 to the upper layer.

In one embodiment, the PDCP layer processing unit 130 may determinewhether data packets waiting for reordering and received from the RLClayer processing unit 120 have been transmitted in connection withre-establishment on the m-RLC layer 121 for the master base station 200Aand control the reordering timer depending on the determination. Forexample, in the case where the data packets waiting for reordering havenot been transmitted in connection with the re-establishment on them-RLC layer 121 for the master base station 200A, in other words, in thecase where the re-establishment has been performed on the s-RLC layer122 or in normal cases where the re-establishment is not performed, uponexpiration of the reordering timer, the PDCP layer processing unit 130transmits the data packets waiting for reordering and received from theRLC layer processing unit 120 to an upper layer.

On the other hand, in the case where the data packets waiting forreordering have been transmitted in connection with the re-establishmenton the m-RLC layer 122 for the master base station 200A, the PDCP layerprocessing unit 130 performs reordering for handover or reconnection.Specifically, the PDCP layer processing unit 130 uses a reception windowto reorder data packets received with the re-establishment and datapackets newly transmitted after deletion of the split bearer. If thereis a missing one in the data packets received with the re-establishment,the PDCP layer processing unit 130 does not update the reception window,and if there is a missing one in the newly transmitted data packets, thePDCP layer processing unit 130 updates the reception window. If the PDCPlayer processing unit 130 receives the data packet falling in thereception window, the PDCP layer processing unit 130 updates thereception window with the sequence number of the received data packet,and if the PDCP layer processing unit 130 receives the data packet beingout of the reception window, the PDCP layer processing unit 130 discardsthe data packet. In this manner, even if the RLC SDUs waiting forreception from the master base station 200A have been discarded, thePDCP layer processing unit 130 can use the reordering timer to deliverthe RLC SDUs received with the re-establishment on the s-RLC layer 122to an upper layer.

FIG. 11 is a flowchart for illustrating split bearer deletion at the RLClayer processing unit according to the first embodiment of the presentinvention.

As illustrated in FIG. 11, at step S101, the master base station 200Aconfigures split bearer for the user equipment 100. The split bearerconfiguration command may be indicated by RRC signaling, for example.

At step S102, the master base station 200A indicates the user equipment100 to delete the split bearer. The split bearer deletion command may beindicated by RRC signaling, for example.

At step S103, the RLC layer processing unit 120 performsre-establishment on the s-RLC layer 122. Specifically, the RLC layerprocessing unit 120 assembles as many RLC SDUs as possible from RLC PDUswaiting for reordering at the s-RLC layer 122 and delivers them to thePDCP layer processing unit 130.

At step S104, the RLC layer processing unit 120 releases the s-RLC layer122.

FIG. 12 is a flowchart for illustrating split bearer deletion at thePDCP layer processing unit according to the first embodiment of thepresent invention.

As illustrated in FIG. 12, at step S201, the master base station 200Aconfigures split bearer for the user equipment 100. The split bearerconfiguration command may be indicated by RRC signaling, for example.

At step S202, upon receiving the split bearer deletion command from themaster base station 200A, in response to the split bearer deletioncommand, the PDCP layer processing unit 130 receives RLC SDUs assembledfrom RLC PDUs waiting for reordering from the RLC layer processing unit120.

At step S203, the PDCP layer processing unit 130 determines whether thereceived RLC SDUs have been transmitted in connection withre-establishment on the m-RLC layer 121. If the received RLC SDUs havebeen transmitted in connection with re-establishment on the m-RLC layer121 (S203: Yes), at step S204, the PDCP layer processing unit 130performs reordering for handover or reconnection.

On the other hand, if the received RLC SDUs have not been transmitted inconnection with the re-establishment on the m-RLC layer 121 (S203: No),at step S205, the PDCP layer processing unit 130 performs reordering forthe split bearer. Specifically, if the sequence numbers of the receivedRLC SDUs are out of sequence, the PDCP layer processing unit 130activates the reordering timer. Then, if a missing one of the RLC SDUshas not been received by expiration of the reordering timer, the PDCPlayer processing unit 130 assembles PDCP SDUs from the RLC SDUs (PDCPPDUs) waiting for reordering and delivers them to an upper layer.

Next, uplink transmission direction change operations in split bearersaccording to the second embodiment of the present invention aredescribed with reference to FIGS. 13-14. As stated above with referenceto FIG. 5, in conventional uplink transmission direction changeoperations, data packets (RLC PDUs #0 to #3 in the s-RLC layer 122 inthe example illustrated in FIG. 5) distributed from the PDCP layer in apre-changed transmission direction are transmitted to a pre-changed basestation 200 (the secondary base station 2003 in the example illustratedin FIG. 5) even after changing the uplink transmission direction, andthere is a likelihood of unsuccessful reception. According to the secondembodiment of the present invention, if the uplink transmissiondirection has been changed in the split bearer, the RLC layer processingunit 120 and the PDCP layer processing unit 130 perform re-establishmenton the RLC layer and the PDCP layer, respectively. As a result, the userequipment 100 can stop transmitting the data packets distributed in thepre-changed transmission direction and transmitting the untransmitteddata packets in the post-changed base station 200 in sequence.

In the second embodiment, when the transmission direction of uplink datapackets is changed in the split bearers configured for the master basestation 200A and the secondary base station 200B, the RLC layerprocessing unit 120 performs re-establishment on the m-RLC layer 121 forthe master base station 200A and the s-RLC layer 122 for the secondarybase station 200B, and the PDCP layer processing unit 130 performs there-establishment on the PDCP layer 131. For example, in the LTEstandard, upon receiving an uplink transmission direction change commandfrom the master base station 200A or the secondary base station 200B,the RLC layer processing unit 120 performs the re-establishment on them-RLC layer 121 and the s-RLC layer 122, and the PDCP layer processingunit 130 performs the re-establishment on the PDCP layer 131.

In one embodiment, the RLC layer processing unit 120 and the PDCP layerprocessing unit 130 may perform the re-establishment on the RLC layers121, 122 and the PDCP layer 131 to stop transmitting uplink data packetsdistributed before changing the transmission direction and transmit thetransmission-stopped uplink data packets to a base station in apost-changed transmission direction. In this case, for example, the RLClayer processing unit 120 and the PDCP layer processing unit 130 mayperform the re-establishment on the RLC layers 121, 122 and the PDCPlayer 131 to transmit the transmission-stopped uplink data packets insequence of the sequence numbers. As a result, the untransmitted uplinkdata packets can be transmitted to the post-changed base station 200 insequence, and data packets having earlier sequence numbers arrive at thepost-changed base station 200 before, which can avoid a likelihood thatdata packets having later sequence numbers may be discarded as being outof a reception window.

In one embodiment, when a re-establishment command is received from thebase station 200 for the pre-changed transmission direction, the RLClayer processing unit 120 and the PDCP layer processing unit 130 mayperform the re-establishment. Specifically, there are some cases wherethe base station 200 continues receiving data packets even afterchanging the transmission direction and the re-establishment isunnecessary. Accordingly, in the case where the base station 200 for thepre-changed transmission direction can receive data packets even afterchanging the transmission direction, the base station 200 may indicateto the user equipment 100 whether the re-establishment is necessary. Forexample, only if the base station 200 for the pre-changed transmissiondirection indicates the re-establishment, the RLC layer processing unit120 and the PDCP layer processing unit 130 may the re-establishment andmay not perform the re-establishment in the other cases. Alternatively,only if the base station 200 for the pre-changed transmission directionindicates that the re-establishment should not be performed, the RLClayer processing unit 120 and the PDCP layer processing unit 130 may notperform the re-establishment and in the other cases, may perform there-establishment. According to this embodiment, it is possible to avoidunnecessary re-establishment.

Also in one embodiment, the re-establishment may be performed on onlyuplink communication. When the uplink transmission direction is changedin the split bearer, the RLC layer processing unit 120 and the PDCPlayer processing unit 130 may perform the re-establishment on only anuplink communication related portion for the RLC layers 121, 122 and thePDCP layer 131. If the re-establishment is also performed on a downlinkcommunication related portion, the downlink communication is temporarilyinterrupted, which can be avoided. Specifically, the user equipment 100may perform the re-establishment on only portions related to thetransmitting side in the RLC layer and the PDCP layer, and the basestation 200 may perform the re-establishment on only portions related tothe receiving side in the RLC layer and the PDCP layer.

FIG. 13 is a flowchart for illustrating an uplink transmission directionchange operation in the user equipment according to the secondembodiment of the present invention.

As illustrated in FIG. 13, at step S301, in response to a configurationcommand from the anchor base station 200, the user equipment 100configures a split bearer for the non-anchor base station 200. The splitbearer configuration command may be indicated by RRC signaling, forexample.

At step S302, the user equipment 100 receives an uplink transmissiondirection change command. The uplink transmission direction changecommand may be indicated by RRC signaling, for example.

At step S303, the RLC layer processing unit 120 and the PDCP layerprocessing unit 130 perform re-establishment on the m-RLC layer 121 andthe s-RLC layer 122 and the PDCP layer 131, respectively. In oneembodiment, the RLC layer processing unit 120 and the PDCP layerprocessing unit 130 may perform the re-establishment on the RLC layers121, 122 and the PDCP layer 131 to stop transmitting uplink data packetsdistributed before changing the transmission direction and transmit thetransmission-stopped uplink data packets to the base station 200 for thepost-changed transmission direction in the order of the sequencenumbers.

FIG. 14 is a block diagram for illustrating an arrangement of the basestation according to the second embodiment of the present invention. Thebase station according to this embodiment has an uplink reordering timerin the PDCP layer and upon changing the uplink transmission direction inthe split bearer, activates the uplink reordering timer and suspendsdelivering data packets received from the user equipment 100 to an upperlayer before the sequence number becomes in sequence during activationof the uplink reordering timer.

As illustrated in FIG. 14, the base station 200 has a transmission andreception unit 210, an RLC layer processing unit 220 and a PDCP layerprocessing unit 230.

The transmission and reception unit 210 communicates with the userequipment 100 in dual connectivity. Specifically, the transmission andreception unit 210 transmits and receives various radio channels, suchas uplink/downlink control channels and uplink/downlink data channels,to/from the user equipment 100.

The RLC layer processing unit 220 has an RLC layer 221 to communicatewith the user equipment 100. In uplink communication in the splitbearer, the RLC layer processing unit 220 assembles RLC SDUs from RLCPDUs received from the user equipment 100 via a lower layer and if thebase station 200 serves as an anchor node in the split bearer, transmitsthe assembled RLC SDUs to its own PDCP layer processing unit 230. On theother hand, if the base station 200 does not serve as the anchor node inthe split bearer, the base station 200 transmits the assembled RLC SDUsto the PDCP layer processing unit 230 in the anchor base station 200.

The PDCP layer processing unit 230 has a PDCP layer 231 to communicatewith the user equipment 100. In uplink communication in the splitbearer, if the base station 200 serves as the anchor node in the splitbearer, the PDCP layer processing unit 230 reorders RLC SDUs receivedfrom its own RLC layer processing unit 220 and RLC SDUs received fromthe RLC layer processing unit 220 in a different base station 200, forwhich the split bearer is configured, to assemble PDCP SDUs and transmitthem to an upper layer.

The PDCP layer processing unit 230 has an uplink reordering timer andwhen the transmission direction of uplink data packets is changed in thesplit bearer configured for the user equipment 100, activates the uplinkreordering timer. If the sequence numbers of the uplink data packetsreceived from the user equipment 100 is out of sequence, the PDCP layerprocessing unit 230 suspends transmitting the received uplink datapackets to an upper layer.

In one embodiment, if the sequence numbers of the uplink data packetsreceived from the user equipment 100 remain out of sequence byexpiration of the uplink reordering timer, the PDCP layer processingunit 230 may transmit the received uplink data packets to the upperlayer. Specifically, if the PDCP layer processing unit 230 has failed toreceive a missing one of the uplink data packets from the user equipment100 before expiration of the uplink reordering timer, the PDCP layerprocessing unit 230 abandons reception of the missing uplink data packetand transmits the uplink data packets waiting for reordering to theupper layer.

On the other hand, if the uplink data packet corresponding to themissing sequence number is received, the PDCP layer processing unit 230stops the uplink reordering timer and transmits the received uplink datapacket to the upper layer. Specifically, if the missing uplink datapacket is received from the user equipment 100 before expiration of theuplink reordering timer, the PDCP layer processing unit 230 reorders thereceived uplink data packet and the uplink data packets waiting forreordering and transmits them to the upper layer.

For differences between the reordering timer and the uplink reorderingtimer as defined in the above-stated split bearer, the former is alwaysused for reordering while the latter is used only at a certain timing,that is, a timing of changing the uplink transmission direction. Inother words, upon detecting a missing sequence number for uplink datapackets received after the uplink reordering timer has expired orstopped, the PDCP layer processing unit 230 determines that the uplinkdata packet corresponding to the missing sequence number has beendiscarded in the user equipment 100.

Note that the above-stated first and second embodiments may be usedseparately or in combination. For example, if the split bearer isapplied, the first embodiment may be to downlink communication, and thesecond embodiment may be applied to uplink communication. Specifically,when the split bearer configured for the secondary base station 200B isdeleted in the downlink communication, the user equipment 100 performsre-establishment on the s-RLC layer 122 for the secondary base station200B and releases the split bearer after execution of there-establishment. On the other hand, when the transmission direction ofuplink data packets is changed in uplink communication in the splitbearer configured for the master base station 200A and the secondarybase station 200B, the user equipment 100 may perform there-establishment on the m-RLC layer 121 for the master base station200A, the s-RLC layer 122 for the secondary base station 200B and thePDCP layer 131.

Although the embodiments of the present invention have been described indetail, the present invention is not limited to the above-statedspecific embodiments, and various modifications and variations can bemade within the spirit of the present invention as recited in claims.

This international patent application claims the benefit of prioritybased on Japanese Priority Application No. 2014-160761 filed on Aug. 6,2014, the entire contents of which are hereby incorporated by reference.

LIST OF REFERENCE SYMBOLS

-   -   10: radio communication system    -   100: user equipment    -   110, 210: transmission and reception unit    -   120, 220: RLC layer processing unit    -   130, 230: PDCP layer processing unit    -   200: base station

1. User equipment, comprising: a transmission and reception unitconfigured to communicate with a master base station and a secondarybase station simultaneously in dual connectivity; an RLC (Radio LinkControl) layer processing unit configured to have an RLC layer for themaster base station and an RLC layer for the secondary base station; anda PDCP (Packet Data Convergence Protocol) layer processing unitconfigured to have a PDCP layer transmitting and receiving data to/fromthe RLC layer for the master base station and the RLC layer for thesecondary base station, wherein when a split bearer configured for thesecondary base station is deleted, the RLC layer processing unitperforms re-establishment on the RLC layer for the secondary basestation and releases the split bearer after execution of there-establishment.
 2. The user equipment as claimed in claim 1, whereinthe RLC layer processing unit transmits data packets staying in the RLClayer for the secondary base station in the re-establishment to the PDCPlayer processing unit.
 3. The user equipment as claimed in claim 2,wherein the staying data packets are RLC PDUs waiting for reordering inthe RLC layer for the secondary base station, and the RLC layerprocessing unit assembles RLC SDUs from the RLC PDUs waiting forreordering and transmits the RLC SDUs to the PDCP layer processing unit.4. The user equipment as claimed in claim 1, wherein if releasing thesplit bearer is deletion on a per EPS (Evolved Packet System) bearerbasis, the RLC layer processing unit deletes the indicated EPC bearerwithout performing the re-establishment on the RLC layer for thesecondary base station.
 5. The user equipment as claimed in claim 1,wherein the PDCP layer processing unit has a reordering timer activatedin response to detecting that data packets are out of sequence, and uponexpiration of the reordering timer, the PDCP layer processing unittransmits data packets to an upper layer, the data packets waiting forreordering and being received from the RLC layer processing unit.
 6. Theuser equipment as claimed in claim 5, wherein the PDCP layer processingunit determines whether the data packets waiting for reordering andreceived from the RLC layer processing unit have been transmitted inconnection with the re-establishment on the RLC layer for the masterbase station and controls the reordering timer depending on thedetermination.
 7. The user equipment as claimed in claim 6, wherein ifthe data packets waiting for reordering have not been transmitted inconnection with the re-establishment on the RLC layer for the masterbase station, upon expiration of the reordering timer, the PDCP layerprocessing unit transmits the data packets waiting for reordering andreceived from the RLC layer processing unit to an upper layer.
 8. Theuser equipment as claimed in claim 6, wherein if the data packetswaiting for reordering have been transmitted in connection with there-establishment on the RLC layer for the master base station, the PDCPlayer processing unit performs reordering for handover or reconnection.9. The user equipment as claimed in claim 1, wherein when a transmissiondirection of uplink data packets is changed in split bearers configuredfor the master base station and the secondary base station, the RLClayer processing unit performs the re-establishment on the RLC layer forthe master base station and the RLC layer for the secondary basestation, and the PDCP layer processing unit performs there-establishment on the PDCP layer.
 10. The user equipment as claimed inclaim 9, wherein the re-establishment configures the RLC layer and thePDCP layer to stop transmitting uplink data packets after changing thetransmission direction and transmit the transmission-stopped uplink datapackets to a base station for the post-changed transmission direction.11. The user equipment as claimed in claim 2, wherein if releasing thesplit bearer is deletion on a per EPS (Evolved Packet System) bearerbasis, the RLC layer processing unit deletes the indicated EPC bearerwithout performing the re-establishment on the RLC layer for thesecondary base station.
 12. The user equipment as claimed in claim 3,wherein if releasing the split bearer is deletion on a per EPS (EvolvedPacket System) bearer basis, the RLC layer processing unit deletes theindicated EPC bearer without performing the re-establishment on the RLClayer for the secondary base station.
 13. The user equipment as claimedin claim 2, wherein the PDCP layer processing unit has a reorderingtimer activated in response to detecting that data packets are out ofsequence, and upon expiration of the reordering timer, the PDCP layerprocessing unit transmits data packets to an upper layer, the datapackets waiting for reordering and being received from the RLC layerprocessing unit.
 14. The user equipment as claimed in claim 3, whereinthe PDCP layer processing unit has a reordering timer activated inresponse to detecting that data packets are out of sequence, and uponexpiration of the reordering timer, the PDCP layer processing unittransmits data packets to an upper layer, the data packets waiting forreordering and being received from the RLC layer processing unit. 15.The user equipment as claimed in claim 4, wherein the PDCP layerprocessing unit has a reordering timer activated in response todetecting that data packets are out of sequence, and upon expiration ofthe reordering timer, the PDCP layer processing unit transmits datapackets to an upper layer, the data packets waiting for reordering andbeing received from the RLC layer processing unit.
 16. The userequipment as claimed in claim 7, wherein if the data packets waiting forreordering have been transmitted in connection with the re-establishmenton the RLC layer for the master base station, the PDCP layer processingunit performs reordering for handover or reconnection.
 17. The userequipment as claimed in claim 2, wherein when a transmission directionof uplink data packets is changed in split bearers configured for themaster base station and the secondary base station, the RLC layerprocessing unit performs the re-establishment on the RLC layer for themaster base station and the RLC layer for the secondary base station,and the PDCP layer processing unit performs the re-establishment on thePDCP layer.
 18. The user equipment as claimed in claim 3, wherein when atransmission direction of uplink data packets is changed in splitbearers configured for the master base station and the secondary basestation, the RLC layer processing unit performs the re-establishment onthe RLC layer for the master base station and the RLC layer for thesecondary base station, and the PDCP layer processing unit performs there-establishment on the PDCP layer.
 19. The user equipment as claimed inclaim 4, wherein when a transmission direction of uplink data packets ischanged in split bearers configured for the master base station and thesecondary base station, the RLC layer processing unit performs there-establishment on the RLC layer for the master base station and theRLC layer for the secondary base station, and the PDCP layer processingunit performs the re-establishment on the PDCP layer.
 20. The userequipment as claimed in claim 5, wherein when a transmission directionof uplink data packets is changed in split bearers configured for themaster base station and the secondary base station, the RLC layerprocessing unit performs the re-establishment on the RLC layer for themaster base station and the RLC layer for the secondary base station,and the PDCP layer processing unit performs the re-establishment on thePDCP layer.